1. Field of the Invention
The present invention relates to a color conversion apparatus that converts a video signal in equipment that deals with a video signal in a form separated into luminance and chroma signals, so that color information expressed by the video signal does not exceed the color reproduction range of primary color signals.
2. Description of the Related Art
Color information is usually expressed in a signal form using the CIE-recommended L*u*v* or L*a*b uniform color space or in a signal form called luminance and color difference signals such as (Y, U, V) and (Y, R-Y, B-Y) widely used in video equipment. All these expressions consist of a signal expressing lightness and a chromaticity vector expressing hue and saturation. Since lightness, hue, and saturation are easily perceived and understood by humans, they are easily varied for adjustment. Further, the chromaticity components that are hard to be perceived by human vision because of their high spatial frequency can be discarded by band restriction, so that the transmission band and memory capacity can be reduced. All these signals are generally called luminance and chroma signals.
On the other hand, there exists a signal form of so called primary color signals such as R (red), G (green), and B (blue) signals. These signals correspond to primary colors of physical light. They do not correspond to human vision, but are easy to handle for designing equipment, so that they are used in equipments not requiring color adjustment but emphasizing image quality, in particular, equipments where compatibility with computers is essential.
Further, these two types of signal forms often coexist in a single piece of equipment. For example, at a unit of adjusting color and gradation in an equipment that mainly processes primary color signals, primary color signals are locally converted into luminance and chroma signals to be processed and then reconverted into primary color signals. The conversion and reconversion are sometimes performed several times in a single piece of equipment.
However, color reproduction ranges expressed by these signal forms are different from each other. Therefore, while conversion from a signal form having a narrower color reproduction range into a signal form having a wider color reproduction range is performed without any limitation, color and gradation overflow in conversion from a signal form having a wider color reproduction range into a signal form having a narrower color reproduction range resulting in the degradation of image quality. Moreover, between two signal forms A and B, A may have a wider color reproduction range than B in some hue, and B may have a wider color reproduction range than B in another hue.
Luminance and chroma signals, which have a wider color reproduction range than RGB primary signals, are often used in a unit requiring color adjustment as described above. An adjusted color is expressed in luminance and chroma signals, but may be beyond the color reproduction range of RGB signals. If the color can not be expressed by RGB signals, then it can not be output by a physical apparatus, so that overflow in color and gradation occurs.
In case of television, the adjustment of raising saturation to make a color dense is performed by expanding the amplitude of color differences by increasing the chroma gain. This method is effective for input colors having a low saturation. However, if a color having great color differences, then even if the adjusted color does not overflow in luminance and color difference signals, it may exceed predetermined maximum levels or become negative when they are converted into RGB signals for activating a CRT. The converted signals are clipped in practice, so that overflow in color and gradation described above occurs causing changes in hue and saturation and deterioration of gradation due to false contours.
Prior arts have been obliged to respond to this problem by setting contrast and saturation low. But it has been difficult to eliminate overflow, and inadequate dark colors have been displayed without sufficiently utilizing the color reproduction range.
Next, the above problem is described in more detail with reference to FIG. 13 that shows a color adjustment circuit. FIG. 13 illustrates a configuration of an adjustment circuit that performs adjustment of lightness, hue, and saturation for luminance and color difference signals. A reference numeral 90 denotes a gain adjustment circuit, 91 denotes a matrix circuit for adjusting the hue and saturation of color difference signals, and 92 denotes an RGB conversion circuit that converts the adjusted luminance and color difference signals into RGB signals.
The operation of the adjustment circuit is exemplified in the following with consideration of signal levels. Relationships among luminance and color difference levels and RGB levels in the NTSC system are given by equations (1), (2), and (5). EQU Y=0.3.multidot.R+0.59.multidot.G+0.11.multidot.B (1) EQU G-Y=-0.5085.multidot.(R-Y)-0.1864.multidot.(B-Y) (2) EQU (R-Y)'=a0.multidot.(R-Y)+a1.multidot.(B-Y), (B-Y)'=a2.multidot.(R-Y)+a3.multidot.(B-Y). (3) EQU a0=h.multidot.cos w a1=h.multidot.sin w a2=-h.multidot.sin w a3=h.multidot.cos w (4) EQU R=(R-Y)+Y G=-0.5085.multidot.(R-Y)-0.1864.multidot.(B-Y)+Y B=(B-Y)+Y(5)
First, adjustments of the lightness of an image are shown in Tables 1-A and 1-B. The input colors 1 and 2 are yellow (Y=0.69, R-Y=0.06, B-Y=-0.49) and bluish cyan (Y=0.666, R-Y=-0.466, B-Y=0.284), and the luminance levels of both colors are around 0.7, an intermediate lightness. Now, if the luminance of both signals are increased into 1.3 times the original ones, the Y levels become a little under 0.9, so that they do not overflow. However, when converted into RGB signals, the RGB levels of the input color 1 are all below 1, but the G and B levels of the input color 2 are over 1 so that they exceed the reproduction range. In practice, the RGB signals beyond level 1 are limited below 1 somewhere in the processing system, and a saturated color shown in the last column of Table 1-B, where G=B=1, is reproduced. The Y level of the saturated color is 0.822, a little under the desired level 0.866, and the levels of the color difference signals, in particular the amplitude of B-Y, become greatly reduced, so that the original bluish cyan changes into purer cyan.
TABLE 1-A ______________________________________ Input color 1 1.3 .multidot. Y ______________________________________ R 0.75 0.96 G 0.75 0.96 B 0.2 0.41 Y 0.69 0.90 R-Y 0.06 0.06 G-Y 0.06 0.06 B-Y -0.49 -0.49 Yellow Yellow ______________________________________
TABLE 1-B ______________________________________ Input color 2 1.3 .multidot. Y Limit ______________________________________ R 0.2 0.40 0.40 G 0.85 1.05 1.0 B 0.95 1.15 1.0 Y 0.666 0.866 0.82 R-Y -0.466 -0.466 -0.42 G-Y 0.184 0.184 0.18 B-Y 0.284 0.284 0.18 Bluish cyan Cyan ______________________________________
Examples in case lightness is changed have been described above. If saturation is expanded, or even if hue is uniformly rotated by processing the color difference signals through a matrix means, overflow at the RGB levels occurs depending on the color. The reason is that the reproduction range of lightness and saturation in the RGB space greatly varies with hue. Not only the RGB levels often exceed their reproducible maximum levels in the color adjustment operation but also sometimes become negative.
Overflow in color reproduction in conversion into primary color signals have been described above with examples of color adjustment. More generally, the same kind of image degradation occurs whenever conversion between signals having different color reproduction ranges is performed.
As mentioned before, prior arts have responded to this problem by moderately setting contrast and saturation to reduce bad effects. An additional method proposed that a reproduced color is determined as a point where the straight line connecting a color point outside a color reproduction range with white color point intercepts the boundary of a color reproduction range (Japanese Pat. Kokai Sho 61-288690).
As described above, when luminance and chroma signals or luminance and color difference signals are converted into primary color signals, the color reproduction range of primary color signals is exceeded, that is, one or more of the primary signals often exceed the reproducible maximum levels or become negative, so that the converted primary signals are clipped with the maximum or zero level. As a result, color reproduction overflows, false contours occur with deterioration of gradation, and hue, saturation, and lightness change. The fact that luminance and chroma signals or luminance and color difference signals exceed the color reproduction range of primary color signals often occurs when luminance, hue, lightness and the like are adjusted. The fact also occurs even if color adjustments are not performed when the color reproduction range of primary color signals is relatively narrow.
Prior arts have tried to prevent overflow in color reproduction based on experience by lowering saturation and contrast in luminance and chroma signals and luminance and color difference signals. But the saturation of not-overflowing colors also declines and the whole image becomes dark, so that accurate color reproduction can not be expected, and overflow in particular colors still occur unless saturation is set exceedingly low.
Further, the Japanese Pat. Kokai Sho 61-288690, which aims to solve these problems, calculates a color reproduction range of complex shape in the chromaticity diagram, judges which of the six color regions R, G, B, C, M, Y an input color belongs to, judges if the input color exceeds the color reproduction range, and obtains the intersection of the boundary of the color range and the obtained hue. Further, the color reproduction range in the chromaticity diagram varies with lightness, so that obtaining the color reproduction range requires very complex calculation. Therefore, the method is hard to implement by hardware, and even if it is implemented by software, the processing time becomes very long, so that implementation in an actual apparatus has been very difficult.